1. Field of the Invention
This invention relates to gelable and gelled compositions and more particularly to gelable and gelled compositions containing agar.
2. Prior Art
Many gel-like compositions have been described in the literature for a variety of applications. The gelable and gelled compositions described herein, however, are significantly different from various prior art formulations categorized as viscous, jelly-like, rubbery, pastes, creams, etc. These gelled compositions comprise true gels which have a rigidity and form common to solids. Viscosity is a meaningless term in describing these gelled solutions because the gel formers form a three-dimensional network which contains the liquid components and this three-dimensional network is broken down in attempts to measure viscosity. One method of characterizing these gelled compositions is in terms of their break strengths and elasticities.
Gelable dispersions are also described herein which comprise dispersions of gel formers in a liquid medium which is to be converted to the gelled state. Since all of these dispersions contain agar, gelation is accomplished by heating the dispersion above the gel-critical temperature and subsequently cooling the dispersion below the gelation or setting temperature.
The compositions described herein contain agar, which is a natural gel-forming agent derived from the red seaweed family. Agar gels traditionally have relatively low-setting temperatures contrasted to gels formed with other gel formers. In many gel applications, it is particularly desirable to take advantage of agar's low-setting temperature.
One example of a situation where it is advantageous to use the low-setting agar in gelled liquids is where the gelled liquid serves as a carrier for a therapeutic agent which is intended for topical application to the human body. If agar gelable compositions could be used for this purpose, application of the composition to the patient's body could be greatly simplified because of the low-setting temperature of agar gels. In other words, the gelled compositions could be formed upon the patient's body, which has heretofore been impossible due to the high setting temperatures of presently available gelable compositions.
It would also be a particular advantage to be able to use low-setting agar gels in photoprocessing applications. In photoprocessing, gelable dispersions are heated above their critical temperature and then extruded onto exposed photosensitive media whereupon they set upon cooling and are stripped off after processing. In many film processing applications, particularly color film, it is critically important that the gelable material applied to the film have as low a temperature as possible. The high-setting temperatures of formerly available gelable dispersions had a tendency to destroy good color balance due to overprocessing of the top layer in color film processing.
Despite their appeal because of their low-gelling temperature, solutions gelled solely with agar have not been used in many possible applications because they have simply not had the other properties required for these applications. For example, solutions gelled solely with agar are very inelastic and brittle and therefore not suitable for extrusion and removal from a photosensitive medium nor are they suitable for topical application of therapeutic solutions. For extrusion, the agar gels have another drawback in that at the elevated extrusion temperatures these gelable dispersions have extremely low viscosities which give them a watery consistency unsuitable for extrusion. Therefore, it has long been desirable to modify agar gels to improve these properties.
While many attempts have been made to improve these properties, none have been successful to date. The addition of thickeners and viscousing agents to improve the viscosity of the gelable dispersions at extrusion temperatures has resulted in gelled solutions with very low break strengths. Combining other gelling agents with agar has customarily resulted in the gel formers tending to compete with each other providing adverse results.